1. Field of the Invention
The present invention relates to a transmitter that employs an OTDM (Optical Time Division Multiplexing) system and a CS (Carrier-Suppressed)-RZ (Return to Zero) modulation system. More precisely, the present invention relates to technology for detecting and adjusting a carrier-wave phase shift in an optical time division multiplexing device that employs an optical time division multiplexing system and a CS-RZ modulation system.
2. Description of Related Art
Conventionally, optical time division multiplexing systems are known as signal multiplexing technology suitable for high-speed, high-capacity optical transmission systems. In addition, CS-RZ modulation systems are known as modulation technology suited to such optical transmission systems.
Optical time division multiplexing systems are systems for multiplexing optical signals of a plurality of channels by switching the transmission channel at fixed times. Optical signals of two channels can be multiplexed by switching the transmission channel for each one-bit cycle, for example.
The CS-RZ modulation system is a modulation system produced by adding CS technology to an RZ system. An RZ system is a system in which an area with a signal value of zero is provided between each of the signal bits. On the other hand, a system in which a zero area is not provided is known as a NRZ (Non Return to Zero) system. An RZ system has the advantage that timing control is simple in comparison with that of an NRZ system. Meanwhile, CS technology is technology in which the phase of the carrier wave is shifted by a half wavelength between adjacent signal bits. When there is an overlap between adjacent signal bits A and B, the optical intensity of the mixed signal component C of the overlapping part increases most when the phases of the carrier waves forming the signal bits match and the phases cancel each other out most when the phases are shifted by a half wavelength. Therefore, the optical intensity of parts where the adjacent signal bits overlap can be reduced by shifting the carrier-wave phases of adjacent signal bits by a half wavelength and, therefore, the complete separation of these signal bits (that is, obtaining an RZ-system signal waveform) become easy. In a high-speed, high-capacity optical transmission system, the signal-bit interval is desirably shortened. Therefore, CS technology is effective when the RZ system is adopted for such a high-speed, high-capacity optical transmission system.
In order to combine the above optical time division multiplexing system and the CS-RZ system, the signals of the respective channels may be formed from carrier waves the phases of which are shifted by a half wavelength each other. For example, if the signals of two channels is formed from carrier waves the phases of which have been shifted by a half wave, and these signals are time-division-multiplexed, the carrier-wave phases of adjacent signal bits can be always shifted by a half wave. Further, when time-division-multiplexing of four channel signals is performed, the carrier-wave phases of adjacent signal bits can be always shifted by a half wave, for example, by matching the phases of the carrier waves of the first and third channels, matching the phases of the carrier waves of the second and fourth channels and shifting the phases of the carrier waves of the first and the second by a half wave.
As is commonly known, a bit signal is generated by modulating a carrier wave by means of a Mach-Zehnder-type modulator or the like. Therefore, the phase of the carrier wave constituting the bit signal can be controlled by adjusting the timing for modulating the carrier wave. Hence, the carrier wave has an extremely high frequency and, therefore, it is not easy to control the phase of the carrier wave very accurately. In addition, the phase of the carrier wave sometimes fluctuates with a dependence on the temperature and so forth. As a result, technology for measuring the phase difference highly accurately in order to control the carrier-wave phase difference between adjacent signal bits highly accurately is desirable.
The present inventor has already proposed a technology for measuring the carrier-wave phase difference by means of Japanese Patent Application Laid-Open No. 2004-23537. In the technology of the patent application, the phase difference is measured by detecting the optical intensity by overlapping adjacent signal bits (See paragraph 0020, FIG. 1 and so forth of the Japanese application) by means of a planar optical waveguide section 310. As is clear from the above description, when adjacent signal bits are overlapped, the optical intensity of the mixed signal increases most when the phases of the carrier waves forming the signal bits match and the carrier waves cancel each other out most when the phases are shifted by a half wavelength. Therefore, by measuring the optical intensity of the superimposed signals, the phase difference between the carrier waves constituting the signal bits can be detected.
In the case of the technology of the Japanese application, the combinations when adjacent signal bits of signal light that is outputted from a transmitter 100 (See FIG. 1 of the Japanese application) are overlapped are the four types 1•1, 0•1, 1•0, and 0•0. In the following description, it is assumed that the optical intensity is ‘1’ when the bit signal is ‘1’ and that the optical intensity is ‘0’ when the bit signal is ‘0’. When the overlapping signal bits are ‘1•1’, the optical intensity is ‘4’ if there is a match between the phases of the carrier waves of the two signal bits but the optical intensity is ‘0’ if the phases of the carrier waves of the two signal bits are shifted by a half wavelength. Further, when the combined signal bits are ‘1•0’ and ‘0•1’, the optical intensity is ‘1’ irrespective of the phase difference of the carrier waves. In addition, when the combined signal bits are ‘0•0’, the optical intensity is ‘0’ irrespective of the phase difference between the carrier waves. Therefore, supposing that the mark ratio of the signal light (the ratio that exists between ‘0’ and ‘1’) is ½, the average optical intensity detected by the planar optical waveguide section (the PLC section in FIG. 2 of the Japanese application) 310 is (4+1+1+0)/4= 3/2 if the carrier-wave phases of the signal bits match but is (0+1+1+0)/4=½ if the carrier-wave phases of the signal bits are shifted by a half wavelength. In other words, in the technology of the Japanese application, in the case where the carrier-wave phases of the signal bits match and the case where the phases are shifted by a half wavelength, the amount of change in the optical intensity (that is, duty ratio) is three times (that is, 4.8 dB).
The optical intensity change amount is not said to be sufficiently large and it is therefore difficult to detect the carrier wave phase difference accurately.